Apparatus and method for high speed scan for detection and measurement of properties of sub-wavelength defects and artifacts in semiconductor and mask metrology

ABSTRACT

A method of detecting defects or artifacts on or in an object, wherein the defects or artifacts are characterized by a characteristic dimension, the method involving: generating an input beam for illuminating a spot at a selected location on or in the object, wherein the spot has a size L that is substantially larger than the characteristic dimension; deriving a measurement beam and a reference beam from the input beam; directing the measurement beam onto the object as an incident measurement beam that illuminates the spot at that selected location on or in the object to produce a backscattered measurement beam; interfering the backscattered measurement beam with the reference beam to produce an interference beam, the reference beam being oriented relative to the backscattered measurement beam so as to produce a peak sensitivity for a portion of the backscattered measurement beam that emanates from the object at a predetermined diffraction angle; converting the interference beam for that selected location into an interference signal; and using the interference signal for that selected location to determine whether any defects or artifacts characterized by said characteristic dimension are present anywhere within a region on or in the object defined by the spot at that selected location.

This application claims the benefit of U.S. Provisional Application No.60/485,507, filed Jul. 7, 2003.

TECHNICAL FIELD

This invention relates generally to interferometric measurement toolsand methods.

BACKGROUND OF THE INVENTION

Various measurement tools have been developed that measure theintensities of scattered/reflected beams by an object so as to detectdefects and/or artifacts on unpatterned and patterned wafers andlithography masks.

SUMMARY OF THE INVENTION

Some of the novel aspects of the inventions described herein include thefollowing. One novel aspect is the practice of backscattered imaging ofan object comprising the interferometric measurement of conjugatedquadratures of fields of beams backscattered by an object which givesinformation in the form of an amplitude and an interferometric phase.Another novel aspect is the practice of backscattered imaging comprisingjoint measurements of conjugated quadratures of fields of backscatteredbeams by an object based on interferometric measurements. A furthernovel aspect is the practice of backscattered imaging by measuringsimultaneously each component of conjugated quadratures of fields ofbackscattered beams by an array of a large number of spots in or on anobject based on interferometric measurements. Still another novel aspectis the practice of backscattered imaging with simultaneous jointmeasurements of conjugated quadratures of fields of backscattered beamsby a large array of spots in or on an object based on interferometricmeasurements.

Embodiments of the invention generate one-, two-, and three-dimensionalbackscattered images of an object with the measurement of conjugatedquadratures of fields of backscattered beams by an object based oninterferometric measurements. Certain embodiments of the inventionfurther generate one-, two-, and three-dimensional backscattered imagesof an object with joint measurements of conjugated quadratures of fieldsof backscattered beams by an object based on interferometricmeasurements.

In other embodiments of the invention, backscattered images of an objectare generated with simultaneous measurements made of conjugatedquadratures of fields of backscattered beams by an array of a largenumber of spots in or on an object based on interferometric measurementsbut with the components of the conjugated quadratures are not measuredjointly. In yet other embodiments of the invention, backscattered imagesof an object are generated with simultaneous joint measurements made ofconjugated quadratures of fields of backscattered beams by a large arrayof spots in or on an object based on interferometric measurements.

In other embodiments of the invention, backscattered images of an objectare generated with simultaneous measurements of conjugated quadraturesof backscattered fields of polarized measurement beams with thecomponents of the conjugated quadratures measured jointly or notmeasured jointly.

In some aspects, the invention teaches the measurement of the conjugatedquadratures of high frequency spatial components of fields scattered bya defect or by artifacts of an object such as an unpatterned orpatterned wafer or lithography mask wherein the measurement beams maycomprise one or more polarization states.

In other aspects the invention further teaches the measurement of thelocation of defects and artifacts from the interferometric phase givenby the corresponding the conjugated quadratures of high frequencyspatial components of fields scattered by the defect and artifacts of anobject.

In general, in one aspect, the invention features a method of detectingdefects or artifacts on or in an object, therein the defects orartifacts characterized by a characteristic dimension. The methodinvolves: generating an input beam for illuminating a spot at a selectedlocation on or in the object, the spot having a size L that issubstantially larger than the characteristic dimension; deriving ameasurement beam and a reference beam from the input beam; directing themeasurement beam onto the object as an incident measurement beam thatilluminates said spot at that selected location on or in the object toproduce a backscattered measurement beam; interfering the backscatteredmeasurement beam with the reference beam to produce an interferencebeam, the reference beam being oriented relative to the backscatteredmeasurement beam so as to produce a peak sensitivity for a portion ofthe backscattered measurement beam that emanates from the object at apredetermined diffraction angle; converting the interference beam forthat selected location into an interference signal; and using theinterference signal for that selected location to determine whether anydefects or artifacts characterized by the characteristic dimension arepresent anywhere within a region on or in the object defined by the spotat that selected location.

Other embodiments include one ore more of the following features. Thespot size L is at least three times greater than the characteristicdimension or alternatively at least an order of magnitude larger thanthe characteristic dimension. The incident measurement beam is at anangle of incidence θ_(I) with respect to a direction that is normal tothe surface of the object, the predetermined diffraction angle is anangle θ_(D) relative to the direction that is normal to the surface ofthe object, the characteristic dimension is equal to Λ, the incidentmeasurement beam is characterized by a wavelength λ, and the diffractionangle θ_(D) is selected to satisfy the following relationship:Λ[sin(θ_(I))−sin(θ_(D))]=λ. The method also includes performing thesteps of generating, deriving, directing, interfering, and convertingfor each of a sequence of different selected locations on or in theobject, wherein the first-mentioned selected location is one of theplurality of different selected locations. Generating the input beaminvolves generating a first beam at a first wavelength and a second beamat a second wavelength that is different from the first wavelength,wherein the first and second beams are coextensive and share the sametemporal window. For each of a plurality of successive time intervals,introducing a corresponding different shift in a selected parameter ofthe first beam and introducing a different corresponding shift in theselected parameter of the second beam, wherein said selected parametersare selected from a group consisting of phase and frequency. Using theinterference signal to determine whether any defects or artifacts arepresent involves: for each of the plurality of successive timeintervals, measuring a value of the interference signal; from themeasured values of the interference signal for the plurality ofsuccessive time internals, computing the orthogonal components ofconjugated quadratures of fields of the backscattered measurement beam;and using the two computed orthogonal components of conjugatedquadratures of fields of the backscattered measurement beam to determinewhether any defects or artifacts characterized by the characteristicdimension are present within the spot. Each of the first and secondbeams includes a first component and a second component that isorthogonal to the first component, wherein the selected parameter of thefirst beam is the phase of the second component of the first beam, andwherein the selected parameter of the second beam is the phase of thesecond component of the second beam. Alternatively, the selectedparameter of the first beam is the frequency of the first beam, and theselected parameter of the second beam is the frequency of the secondbeam.

Still other embodiments include one or more of the following features.Using the interference signal for that selected location to determinewhether any defects or artifacts are present involves jointly measuringtwo orthogonal components of conjugated quadratures of fields of thebackscattered measurement beam. The method is implemented in aninspection tool wherein the object is a mask or a retilce.Alternatively, the method is implemented in a lithography tool whereinthe object is a wafer or the object is a wafer stage and the artifactsare alignment marks.

In general, in another aspect, the invention features a methodinvolving: focusing an incident measurement beam to a spot thatilluminates a target area on or in the object to produce a backscatteredmeasurement beam, the spot having a size L that is substantially largerthan the characteristic dimension; interfering the backscatteredmeasurement beam with a reference beam to produce an interference beam,said reference beam being oriented relative to the backscattered beam soas to produce a peak sensitivity for a portion of the backscattered beamthat emanates from the object at a predetermined diffraction angle;converting the interference beam to an interference signal; anddetermining from the interference signal whether any defects orartifacts characterized by the characteristic dimension are presentwithin the target area.

In general, in still another aspect, the invention features a methodinvolving: generating a reference beam; generating a measurement beamfor illuminating a spot on or in the object, the spot having a size Lthat is substantially larger than the characteristic dimension; andusing the measurement and reference beams to interferometricallydetermine whether any defects or artifacts characterized by saidcharacteristic dimension are present on or in the object.

An advantage of at least one embodiment of the invention is that one-,two-, and three-dimensional backscattered images of an object aregenerated with the measurement of conjugated quadratures of fields ofbackscattered beams by an object based on interferometric measurements.

Another advantage of at least one embodiment of the invention is thatone-, two-, and three-dimensional backscattered images of an object aregenerated with joint measurements of conjugated quadratures of fields ofbackscattered beams by an object based on interferometric measurements.

Another advantage of at least one embodiment of the invention is thatbackscattered images of an object are generated with simultaneousmeasurements of conjugated quadratures of fields of backscattered beamsby an array of a large number of spots in or on an object ininterferometric measurements but with the components of the conjugatedquadratures are not measured jointly.

Another advantage of at least one embodiment of the invention is thatbackscattered images of an object are generated with simultaneousmeasurements of joint measurements of conjugated quadratures of fieldsof backscattered beams by a large array of spots in or on an object ininterferometric measurements.

Another advantage of at least one embodiment of the invention is that abi- or quad-homodyne detection method can be used to obtain conjugatedquadratures of fields of beams backscattered by a substrate that isbeing imaged.

Another advantage of at least one embodiment of the invention is that avariant of the bi- or quad-homodyne detection method can be used toobtain joint measurements of conjugated quadratures of fields ofbackscattered orthogonally polarized beams by a substrate that is beingimaged.

Another advantage of at least one embodiment of the invention is thatrelative phase shifts between the arrays of reference and measurementbeams can be introduced in certain embodiments of the invention bychanging the frequencies of components of the input beam.

Another advantage of at least one embodiment of the invention is thatrelative phase shifts from a predetermined set of relative phase shiftscan be introduced between the arrays of reference and measurement beamsin certain embodiments of the invention.

Another advantage of at least one embodiment of the invention isbackscattered imaging of a substrate with a sub-wavelength lateralresolution may be obtained with a working distance of the order of a mm.

Another advantage of at least one embodiment of the invention isbackscattered imaging of a substrate with a sub-wavelength lateralresolution may be obtained with a working distance of the order ofmicrons.

Another advantage of at least one embodiment of the invention is thatthe location of defects and artifacts may be obtained from theinterferometric phase given by the corresponding the conjugatedquadratures of high frequency spatial components of fields scattered bythe defect and artifacts of an object.

Another advantage of at least one embodiment of the invention isbackscattered imaging of an interior portion of a substrate with asub-wavelength lateral resolution may be obtained with a workingdistance of the order of a mm.

Another advantage of at least one embodiment of the invention isbackscattered imaging of an interior portion of a substrate with asub-wavelength lateral resolution may be obtained with a workingdistance of the order of microns.

Another advantage of at least one embodiment of the invention is thatthe phases of the input beam components in certain embodiments of theinvention do not affect measured conjugated quadratures of fields.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a diagram of an interferometric system.

FIG. 1 b is a schematic diagram of an embodiment of mask or reticleinspection tool and an embodiment of a wafer/stage inspection portion ofa lithography tool with the exposure optic and source omitted thatcomprises an interferometric metrology system which in turn comprises acatadioptric imaging system.

FIG. 1 c is a diagram of a catadioptric imaging system.

FIG. 1 d is a diagram of a pinhole array beam-splitter.

FIG. 1 e is a schematic diagram of a beam-conditioner configured as atwo-frequency generator and a frequency-shifter.

FIG. 2 a is a schematic diagram of an achromatic astigmatic catadioptricimaging system.

FIG. 2 b is a diagram showing surfaces and corresponding radii of acatadioptric imaging system.

FIG. 2 c is a schematic diagram an astigmatic catadioptric imagingsystem.

FIG. 2 d is a schematic diagram of a section of a catadioptric imagingsystem located near a measurement object.

FIG. 2 e is a schematic diagram of a section of a catadioptric imagingsystem located near a measurement object and imaging an interior sectionof the measurement object.

FIG. 2 f is a schematic diagram an astigmatic catadioptric imagingsystem.

FIG. 3 is a diagram of the angles of incidence and diffraction of beams.

DETAILED DESCRIPTION

An apparatus and method is described for measurement of a high frequencyspatial component of conjugated quadratures of fields of a beamreflected/scattered by a spot of a two-dimensional section in or on ameasurement object, e.g., an unpatterned or patterned wafer orlithography mask. The spot of the two-dimensional section may comprise adefect or artifact wherein a characteristic scale of the defect orartifact is less than the wavelength of the beam and is of the order of½ to ¼ the spatial wavelength of the measured high frequency spatialcomponent. The conjugated quadrature amplitudes of the high frequencyspatial component are measured for the spot of the two-dimensionalsection where the characteristic dimension of the spot is approximatelyan order of magnitude or more larger than the spatial wavelength of thehigh frequency spatial component. The high frequency spatial componentof reflected/scattered fields preferentially contains information aboutproperties and locations of sub-wavelength defects and artifacts of themeasurement object. Information about the locations is contained in theenvelop function of the measured high frequency spatial component and inthe interferometric phase given by the corresponding conjugatedquadratures. The use of a spot with a characteristic dimension that isapproximately an order of magnitude or more larger than the spatialwavelength of the high frequency spatial component in the measurement ofthe conjugated quadrature amplitudes permits a high speed scan of thesection in a survey searching for sub-wavelength defects and/or toobtain information about properties of sub-wavelength defects and/or theartifacts. The apparatus and method for the measurement of highfrequency spatial component comprises interferometric measurement offields of backscattered beams by the object. The operation comprisingmeasurement of conjugated quadratures of backscattered beams has theimportant property that the measurement of the high frequency spatialcomponent is made operating in a dark field mode. The use of a darkfield mode leads to an improved signal-to-noise ratio for measurement ofthe high frequency spatial component. In certain embodiments,information about backscattered fields is obtained for differentpolarization states of the measurement beams. Information about thebackscattered fields is obtained from joint measurements of conjugatedquadratures of the corresponding amplitudes using either a bi- orquad-homodyne detection method or variants thereof. The informationabout the backscattered fields is obtained from joint measurements of2-dimensional arrays of conjugated quadratures in a scanning mode for a100% coverage of a section.

A general description of the technique and procedure of the inventionwill first be given followed by a technical description of embodiments.The technique and procedure are based on the spatial frequency responseof a two dimensional section of a substrate. The two-dimensional sectionmay correspond to a surface of the substrate or to an internaltwo-dimensional section of the substrate. A substrate reflectance can beexpanded into a Fourier series of spatial harmonics with eachrepresenting a spatial frequency component. The conjugated quadraturesof the spatial frequency component represent properties of atwo-dimensional section of the substrate. The conjugated quadratures ofthe spatial frequency component are written as $\begin{matrix}{{{F( {x,y,\Lambda_{x},\Lambda_{y},\zeta_{x},\zeta_{y}} )} = {{A( {x,y,\Lambda_{x},\Lambda_{y},\zeta_{x},\zeta_{y}} )} \times {\cos( {\frac{2\quad\pi\quad x}{\Lambda_{x}} + \zeta_{x}} )}{\cos( {\frac{2\quad\pi\quad y}{\Lambda_{y}} + \zeta_{y}} )}}},} & (1) \\{{\overset{\sim}{F}( {x,y,\Lambda_{x},\Lambda_{y},\zeta_{x},\zeta_{y}} )} = {{\overset{\sim}{A}( {x,y,\Lambda_{x},\Lambda_{y},\zeta_{x},\zeta_{y}} )} \times {\sin( {\frac{2\quad\pi\quad x}{\Lambda_{x}} + \zeta_{x}} )}{\sin( {\frac{2\quad\pi\quad y}{\Lambda_{y}} + \zeta_{y}} )}}} & (2)\end{matrix}$where F(x,y,Λ_(x),Λ_(y)ζ_(x)ζ_(y)) and {tilde over(F)}(x,y,Λ_(x),Λ_(y),ζ_(x),ζ_(y)) are conjugated quadratures of thespatial frequency component, A(x,y,Λ_(x),Λ_(y),ζ_(x),ζ_(y)) andÃ(x,y,Λ_(x),Λ_(y),ζ_(x),ζ_(y)) are the amplitudes of the conjugatedquadratures of the spatial frequency components, and Λ_(x) and Λ_(y) arethe wavelengths of the spatial frequency components in the x and ydirections, respectively, such that 1/Λ_(x) and 1/Λ_(y) are thecorresponding spatial frequencies. Phase offsets ζ_(x) and ζ_(y) inEquations (1) and (2) are determined by the location of the source ofthe corresponding spatial frequency components in or on the twodimensional section of the substrate relative to a Cartesian coordinatesystem.

With respect to the embodiments described herein, the autocorrelationlengths of A(x,y,Λ_(x),Λ_(y)ζ_(x)ζ_(y)) andÃ(x,y,Λ_(x),Λ_(y),ζ_(x),ζ_(y)) in the x and y directions and/or of therespective envelop function are larger, e.g., a factor of three or anorder of magnitude, than the smaller of the spatial wavelengths Λ_(x)and Λ_(y) and generally much less than corresponding dimensions of atwo-dimensional section of a substrate being imaged. The autocorrelationlengths and the associated spatial wavelengths Λ_(x) and Λ_(y) aresubstantially independent parameters controlled by the design of theapparatus used in various embodiments of the invention.

Amplitudes of conjugated quadratures of a spatial frequency component ofa substrate reflectance are measured interferometrically. Theinterferometric measurements will have a peak in detection sensitivityfor defects and/or artifacts that have a characteristic dimension in thex direction of the order of Λ_(x)/2 to Λ_(x)/4 wherein it has beenassumed that Λ_(x)≲Λ_(y) so as to illustrate important properties with asimplified description. Accordingly, the fractional spatial wavelengthΛ_(x)/4 is preferably selected to be of the order of the correspondingcharacteristic dimension of defects and/or artifacts for whichinformation is desired. For other spatial wavelengths, there will be areduced sensitivity.

It is of particular interests to note that in addition to a peak indetection sensitivity for defects and/or artifacts that have acorresponding characteristic dimension of the order of Λ_(x)/4, thedetection sensitivity will be substantially zero for a correspondingzero spatial frequency component. The zero sensitivity for detection ofthe corresponding zero spatial frequency component permits aparticularly important mode of operation, a dark field interferometricmode. In the dark field interferometric mode, the measurements of theamplitudes A(x,y,Λ_(x),Λ_(y),ζ_(x),ζ_(y)) andÃ(x,y,Λ_(x),Λ_(y),ζ_(x),ζ_(y)) are made with a significantly increasedsignal-to-noise ratio (see for example the discussion of statisticalnoise in commonly owned U.S. Pat. No. 5,760,901 (ZI-05) entitled “MethodAnd Apparatus For Confocal Interference Microscopy With BackgroundAmplitude Reduction And Compensation” and U.S. Pat. No. 6,480,285 B1(ZI-08) entitled “Multiple Layer Confocal Interference Microscopy UsingWavenumber Domain Reflectometry And Background Amplitude Reduction AndCompensation” wherein both are by Henry A. Hill, the contents of whichare incorporated herein in their entirety by reference.

It is also of particular interest to note that the interferometric phasecorresponding to amplitude and phase representation of the measuredconjugated quadratures of the high frequency spatial components issensitive in general to displacements of imaged defects and artifacts inthe x and y directions as well as in the z direction. This propertymakes it possible to determine locations of defects and artifactsthrough measurements of a phase which consequently possess theadvantages generally assigned to measuring displacements with lineardisplacement interferometer.

The amplitude of the measured conjugated quadratures of the highfrequency spatial frequency components is sensitive to the scatteringcross section of the respective defects and artifacts. The scatteringcross section is sensitive to the size and composition of the defectsand artifacts.

Because of the characteristic size of the spot being imaged is largerthan the wavelength of the spatial frequency components, the spatialfrequency components will be preferentially diffracted at particularangles in respective planes. For a given optical wavelength X for ameasurement beam, the particular angle at which a spatial frequencycomponent of the substrate reflectance will diffract the beam in acorresponding plane is given by a grating equationΛ[ sin θ_(I)−sin θ_(D)]=λ  (3)where θ_(I) and θ_(D) are angles of incidence and diffraction as showndiagrammatically in FIG. 3. It is evident from Equation (3) that inorder to obtain the smallest Λ for a given optical wavelength λ,θ_(D)≅−θ_(I),  (4)θ_(I)≈1.  (5)

The width Δθ of the beam diffracted by the spatial frequency componentof the substrate reflectance will be determined by an autocorrelationlength l of A(x,y,Λ_(x),Λ_(y),ζ_(x),ζ_(y)) andÃ(x,y,Λ_(x),Λ_(y),ζ_(x),ζ_(y)) as $\begin{matrix}{{\Delta\quad\theta} \cong {\frac{\lambda}{l}\sec\quad{\theta_{R}.}}} & (6)\end{matrix}$The autocorrelation length l is determined by properties of an imagingsystem that determines the dimensions of the spot being imaged.

The defects in or on a patterned wafer are identified by comparing themeasured array of conjugated quadratures A(x,y,Λ_(x),Λ_(y),ζ_(x),ζ_(y))and Ã(x,y,Λ_(x),Λ_(y),ζ_(x),ζ_(y)) with a master array of conjugatedquadratures generated by a master patterned wafer that does not have anydefects.

In order to reduce the density of measurements ofA(x,y,Λ_(x),Λ_(y),ζ_(x),ζ_(y)) and Ã(x,y,Λ_(x),Λ_(y),ζ_(x),ζ_(y))required for a full coverage of a substrate section, the autocorrelationlength l is selected to be larger than Λ_(x) by a factor ζ that is theorder of 10 or more. Thus, a full survey of the substrate section can beobtained for the presence of defects and/or artifacts of characteristicdimensions of Λ_(x)/4 or smaller with a time reduced by a factor of ζ²compared to the time required to execute a full survey of the substratesurface with an image resolution of ≈λ/4.

The sampling frequency for the measurement ofA(x,y,Λ_(x),Λ_(y),ζ_(x),ζ_(y)) and A(x,y,Λ_(x),Λ_(y),ζ_(x),ζ_(y)) ispreferably approximately equal to twice the spatial frequency 1/l, i.e.,2/l. In addition, the sampling spatial frequency is preferably selectedto correspond to the spatial frequency of the pitch of pixel location ofa CCD detector in the x direction.

The survey can locate the positions of defects and/or artifacts forexample by two different data analysis techniques. The amplitudesA(x,y,Λ_(x),Λ_(y),ζ_(x),ζ_(y)) and Ã(x,y,Λ_(x),Λ_(y),ζ_(x),ζ_(y)) inEquations (1) and (2) are indicated as being dependent on the x and ycoordinates and as also containing information about the location of thesource of the spatial frequency components. The spatial resolution ofone of the analysis techniques corresponds to approximately theautocorrelation length l. In this case, the information about thelocation of the source is contained in the envelop function representedby the magnitude$\{ {\lbrack {A( {x,y,\Lambda_{x},\Lambda_{y},\zeta_{x},\zeta_{y}} )} \rbrack^{2} + \lbrack {\overset{\sim}{A}( {x,y,\Lambda_{x},\Lambda_{y},\zeta_{x},\zeta_{y}} )} \rbrack^{2}} \}^{1/2}$of the spatial frequency components.

If a higher spatial resolution is required in an end use applicationafter the defects and/or artifacts are located with a spatial resolutionof approximately l, the locations can be determined made by the secondanalysis technique mod Λ_(x) and Λ_(y) in the x and y directions,respectively, with a spatial resolution of approximately λ/2 to λ/4 whenusing properties of the sinusoidal varying components of the conjugatedquadratures or the interferometric phase of the measured conjugatedquadratures in a amplitude and phase representation thereof.Alternatively, the location information acquired in the survey can beused as input information of a subsequent measurement with aninterferometric confocal microscope such as described in commonly ownedU.S. patent application Ser. No. 10/778,371 (ZI-40) entitled “TransverseDifferential Interferometric Confocal Microscopy;” U.S. ProvisionalApplication No. 60/459,425 (ZI-50) entitled “Apparatus and Method forJoint Measurement of Fields of Scattered/Reflected OrthogonallyPolarized Beams by an Object in Interferometry;” U.S. patent applicationSer. No. 10/816,180 (ZI-50) filed Apr. 1, 2004 and entitled “Apparatusand Method for Joint Measurement Of Fields Of Scattered/Reflected OrTransmitted Orthogonally Polarized Beams By An Object InInterferometry;” U.S. Provisional Application No. 60/460,129 (ZI-51)entitled “Apparatus and Method for Measurement of Fields of ForwardScattered/ Reflected and Backscattered Beams by an Object inInterferometry,” and U.S. patent application Ser. No. 10/816,172,(ZI-51) filed Apr. 1, 2004 and entitled “Apparatus And Method ForMeasurement Of Fields Of Forward Scattered/Reflected and BackscatteredBeams By An Object In Interferometry” of which each are to Henry A. Hilland of which the contents of the two cited provisional patentapplications and of the three patent applications are hereinincorporated in their entirety by reference.

The substrate reflectance will in general be sensitivity to thepolarization of the measurement beam incident on the substrate.Accordingly, the amplitudes of conjugated quadraturesA(x,y,Λ_(x),Λ_(y),ζ_(x),ζ_(y)) and Ã(x,y,Λ_(x),Λ_(y),ζ_(x),ζ_(y)) willbe a function of the polarization of the measurement beam incident onthe substrate. The polarization sensitive properties of the amplitudesof conjugated quadratures A(x,y,Λ_(x),Λ_(y),ζ_(x),ζ_(y)) andÃ(x,y,Λ_(x),Λ_(y),ζ_(x),ζ_(y)) may be measured in various embodiments ofthe invention.

Some of the embodiments described herein may be used to measurepropagation of signals, e.g., thermal or acoustical, in a section byusing a probe beam in addition to the measurement beam where the probebeam precedes the measurement beam by a time τ. The probe beam and themeasurement beams may have different optical wavelengths. An advantageof using those embodiments to measure temporal response of the substrateis the high spatial frequency resolution in the plane of the section.

In the following description of different embodiments, many elements ofthe different embodiments perform like functions and are indicated withthe same numerals in different respective figures of the embodiments.

A general description of embodiments incorporating the present inventionwill first be given for interferometer systems wherein the bi- andquad-homodyne detection methods and variants thereof are used ininterferometer systems for making joint measurements of conjugatedquadratures of fields of beams backscattered by a measurement object.Referring to FIG. 1 a, an interferometer system is showndiagrammatically comprising an interferometer 10, a source 18, abeam-conditioner 22, detector 70, an electronic processor and controller80, and a measurement object or substrate 60. Source 18 is a pulsed orshuttered source that generates input beam 20 comprising one or morefrequency components. Beam 20 is incident on and exits beam-conditioner22 as input beam 24 that comprises a single polarized component or twoorthogonally polarized components. Each of the orthogonally polarizedcomponents comprises two or more different frequency components. Themeasurement beam components of the frequency components of input beam 24are coextensive in space and have the same temporal window function andthe corresponding reference beam components are coextensive in space andhave the same temporal window function although the measurement beamcomponents and the reference beam components may not be spatiallycoextensive.

Reference and measurement beams may be generated in eitherbeam-conditioner 22 from a set of beams or in interferometer 10 for eachof the two or more frequency components of input beam 24. Measurementbeam 30A generated in either beam-conditioner 22 or in interferometer 10is incident on substrate 60. Measurement beam 30B is a returnmeasurement beam generated as either a portion of measurement beam 30Areflected and/or scattered by substrate 60. Return measurement beam 30Bis combined with the reference beam in interferometer 10 to form outputbeam 34.

Output beam 34 is detected by detector 70 to generate either one or moreelectrical interference signals per source pulse for the bi-homodyne orquad-homodyne detection methods or variants thereof as signal 72.Detector 70 may comprise an analyzer to select common polarizationstates of the reference and return measurement beam components of beam34 to form a mixed beam. Alternatively, interferometer 10 may comprisean analyzer to select common polarization states of the reference andreturn measurement beam components such that beam 34 is a mixed beam.

In the practice, known phase shifts are introduced between the referenceand measurement beam components of output beam 34 by two differenttechniques. In the first technique, phase shifts are introduced betweencorresponding reference and measurement beam components for each of thefrequency components of output beam 34 as a consequence of a non-zerooptical path difference between the reference and measurement beam pathsin interferometer 10 and corresponding frequency shifts introduced tothe frequency components of input beam 24 by beam-conditioner 22 and/orsource 18 as controlled by signals 74 and 92 from electronic processorand controller 80. In the second technique, phase shifts are introducedbetween the reference and measurement beam components for each of thefrequency components of input beam 24 by beam-conditioner 22 ascontrolled by signals 74 and 92 from electronic processor and controller80.

There are different ways to configure source 18 and beam-conditioner 22to meet the input beam requirements of the different embodiments of thepresent invention. Examples of beam-conditioners that may be used ineither first or the second technique comprise combinations of a twofrequency generator and phase shifting type of beam-conditioner such asdescribed in commonly owned U.S. Provisional Patent Application No.60/442,858 (ZI-47) entitled “Apparatus and Method for Joint Measurementsof Conjugated Quadratures of Fields of Reflected/Scattered Beams by anObject in Interferometry” and U.S. patent application Ser. No.10/765,369, filed Jan. 27, 2004 (ZI-47) and entitled “Apparatus andMethod for Joint Measurements of Conjugated Quadratures of Fields ofReflected/Scattered and Transmitted Beams by an Object inInterferometry” of which both are to Henry A. Hill and of which thecontents of provisional and non-provisional applications are hereinincorporated in their entirety by reference.

Another example of beam-conditioner that may be used in either the firstor the second technique comprise combinations of multiple frequencygenerators and phase shifting types of beam-conditioners such assubsequently described herein with respect to FIG. 1 e.

With a continuation of the description of different ways to configuresource 18 and beam-conditioner 22 to meet the input beam requirements ofdifferent embodiments of the present invention, source 18 willpreferably comprise a pulsed source. There are a number of differentways for producing a pulsed source [see Chapter 11 entitled “Lasers”,Handbook of Optics, 1, 1995 (McGraw-Hill, New York) by W. Silfvast].Each pulse of source 18 may comprise a single pulse or a train of pulsessuch as generated by a mode locked Q-switched Nd:YAG laser. A singlepulse train is referenced herein as a pulse and a pulse and a pulsetrain are used herein interchangeably.

Source 18 may be configured in certain embodiments of the presentinvention to generate two or more frequencies by techniques such asdescribed in a review article entitled “Tunable, Coherent Sources ForHigh-Resolution VUV and XUV Spectroscopy” by B. P. Stoicheff, J. R.Banic, P. Herman, W. Jamroz, P. E. LaRocque, and R. H. Lipson in LaserTechniques for Extreme Ultraviolet Spectroscopy, T. J. McIlrath and R.R. Freeman, Eds., (American Institute of Physics) p 19 (1982) andreferences therein. The techniques include for example second and thirdharmonic generation and parametric generation such as described in thearticles entitled “Generation of Ultraviolet and Vacuum UltravioletRadiation” by S. E. Harris, J. F. Young, A. H. Kung, D. M. Bloom, and G.C. Bjorklund in Laser Spectroscopy I, R. G. Brewer and A. Mooradi, Eds.(Plenum Press, New York) p 59, (1974) and “Generation of TunablePicosecond VUV Radiation” by A. H. Kung, Appl. Phys. Lett. 25, p 653(1974). The contents of the three cited articles are herein incorporatedin their entirety by reference.

The output beams from source 18 comprising two or more frequencycomponents may be combined in beam-conditioner 22 by beam-splitters toform coextensive measurement and reference beams that are eitherspatially separated or coextensive as required in certain embodiments ofthe present invention. The frequency shifting of the various componentsrequired in certain embodiments of the present invention may beintroduced in source 18 for example by frequency modulation of inputbeams to parametric generators and the phase shifting of reference beamsrelative to measurement beams in beam-conditioner 22 may be achieved byphase shifters of the optical-mechanical type comprising for exampleprisms or mirrors and piezoelectric translators or of theelectro-optical modulator type.

The general description is continued with reference to FIG. 1 a. Inputbeam 24 is incident on interferometer 10 wherein reference beams andmeasurement beams are generated. The reference beams and measurementbeams comprise one or two arrays of reference beams and one or twoarrays of measurement beams, respectively, for measurements usingmeasurement beams that comprise a single polarization state or twoorthogonal polarization states, respectively, wherein the arrays maycomprise arrays of one element. The arrays of measurement beams arefocused on and/or in measurement object 60 and arrays of returnmeasurement beams are generated by reflection/scattering by measurementobject 60. The arrays of reference beams and return measurement beamsare combined by a beam-splitter to form one or two arrays of outputbeams using measurement beams that comprise a single polarization stateor two orthogonal polarization states, respectively. The arrays ofoutput beams are mixed with respect to state of polarization either ininterferometer 10 or in detector 70. The arrays of output beams aresubsequently focused to spots on pixels of a multipixel detector anddetected to generate the array of electrical interference signals 72.

The conjugated quadratures of fields of return measurement beams areobtained by using a single-, double-, bi-, quad-homodyne detectionmethod or variants thereof. The bi- and quad-homodyne detection methodsare described for example in cited U.S. Provisional Patent ApplicationNo. 60/442,858 (ZI-47) and U.S. patent application Ser. No. 10/765,369,filed Jan. 27, 2004 (ZI-47) and entitled “Apparatus and Method for JointMeasurements of Conjugated Quadratures of Fields of Reflected/Scatteredand Transmitted Beams by an Object in Interferometry”. The variants ofthe bi- and quad-homodyne detection methods are described for example incited U.S. Provisional Patent Application No. 60/459,425 (ZI-50) andU.S. Patent Application 10/816,180, filed Apr. 1, 2004 (ZI-50) andentitled “Apparatus and Method for Joint Measurement of Fields ofScattered/Reflected Orthogonally Polarized Beams by an Object inInterferometry”.

For the single-homodyne detection method, input beam 24 comprises asingle frequency component and sets of four or eight measurements of thearray of electrical interference signals 72 is made in non-ellipsometricor ellipsometric measurements, respectively, wherein non-ellipsometricand ellipsometric measurements correspond to measurements made with ameasurement beam 30A comprising a single polarized component ororthogonally polarized components, respectively. For each of themeasurements of the array of electrical interference signals 72 innon-ellipsometric and ellipsometric measurements, known phase shifts areintroduced between each reference beam component and respective returnmeasurement beam component of output beam 34. The subsequent dataprocessing procedure used to extract the conjugated quadratures offields of beams reflected and/or scattered by a substrate issubsequently described herein with respect to the first embodiment ofthe present invention and also for example in commonly owned U.S. Pat.No. 6,445,453 (ZI-14) entitled “Scanning Interferometric Near-FieldConfocal Microscopy” by Henry A. Hill of which the contents areincorporated herein in their entirety by reference.

The double-homodyne detection method which is applicable tonon-ellipsometric measurements uses input beam 24 comprising fourfrequency components and four detectors to obtain measurements ofelectrical interference signals that are subsequently used to obtainconjugated quadratures in non-ellipsometric measurements. Each detectorelement of the four detector elements obtains a different one of thefour electrical interference signal values with the four electricalinterference signal values obtained simultaneously to compute theconjugated quadratures for a field. Each of the four electricalinterference signal values contains only information relevant to oneorthogonal component of the conjugated quadratures. The double-homodynedetection used herein is related to the detection methods such asdescribed in Section IV of the article by G. M D'ariano and M G. A.Paris entitled “Lower Bounds On Phase Sensitivity In Ideal And FeasibleMeasurements,” Phys. Rev. A 49, 3022-3036 (1994). Accordingly, thedouble-homodyne detection method does not make joint determinations ofconjugated quadratures of fields wherein each electrical interferencesignal value contains information simultaneously about each of twoorthogonal components of the conjugated quadratures.

In the adaptation of the double-homodyne detection method toellipsometric measurements, input beam 24 comprises eight frequencycomponents and eight detectors to obtain measurements of eightelectrical interference signals that are subsequently used to obtainconjugated quadratures. Each detector element of the eight detectorelements obtains a different one of the eight electrical interferencesignal values with the eight electrical interference signal valuesobtained simultaneously to compute the conjugated quadratures of fieldsof scattered/reflected orthogonally polarized fields. Each of the eightelectrical interference signal values contains only information relevantto one orthogonal component of one of the two conjugated quadratures.

The bi- and quad-homodyne detection methods obtain measurements ofelectrical interference signals wherein each measured value of anelectrical interference signal contains simultaneously information abouttwo orthogonal components of conjugated quadratures. The two orthogonalcomponents correspond to orthogonal components of conjugated quadraturessuch as described in cited U.S Provisional Patent Application No.60/442,858 (ZI-47) and cited U.S. patent application Ser. No.10/765,369, filed Jan. 27, 2004 (ZI-47) entitled “Apparatus and Methodfor Joint Measurements of Conjugated Quadratures of Fields ofReflected/Scattered and Transmitted Beams by an Object inInterferometry”.

The variants of the bi- and quad-homodyne detection methods obtainmeasurements of electrical interference signals wherein each measuredvalue of an electrical interference signal contains simultaneouslyinformation about two orthogonal components of each of two conjugatedquadratures of fields of scattered/reflected orthogonally polarizedbeams. The two orthogonal components of the two conjugated quadraturescorrespond to orthogonal components of conjugated quadratures such asdescribed in cited U.S Provisional Patent Application No. 60/459,425(ZI-50) and cited U.S. patent application Ser. No. 10/816,180, filedApr. 1, 2004 (ZI-50) and entitled “Apparatus and Method for JointMeasurement of Fields of Scattered/Reflected Orthogonally PolarizedBeams by an Object in Interferometry”.

A first embodiment of the present invention is shown schematically inFIG. 1 b. The schematic diagram in FIG. 1 b is of mask or reticleinspection tool or an diagram of a wafer/stage inspection portion of alithography tool (with the exposure optic and source omitted) used tomeasure locations of artifacts on wafers and/or a stage such asalignment marks. The first embodiment comprises a first imaging systemgenerally indicated as numeral 100, pinhole array beam-splitter 12,detector 70, and a second imaging system generally indicated as numeral110. The second imaging system 110 is low power microscope having alarge working distance, e.g. Nikon ELWD and SLWD objectives and OlympusLWD, ULWD, and ELWD objectives.

The first imaging system 100 is shown schematically in FIG. 1 c. Theimaging system 100 is a catadioptric system such as described incommonly owned U.S. Pat. No. 6,552,852 B2 (ZI-38) and U.S. Pat. No.6,717,736 (ZI-43) of which both are entitled “Catoptric and CatadioptricImaging System,” both applications are to Henry A. Hill, and thecontents of the two cited patents are incorporated herein in theirentirety by reference.

Catadioptric imaging system 100 comprises a section, e.g., a piesection, of catadioptric imaging system 210 shown schematically in FIG.2 a. Elements of catadioptric imaging system 210 shown in FIG. 2 acomprise two different media in order to generate an achromaticanastigmat. Catadioptric imaging system 210 comprises catadioptricelements 240 and 244, beam-splitter 248, concentric lenses 250 and 254,and plano convex lenses 256 and 258. Surfaces 242A and 246A are convexspherical surfaces with nominally the same radii of curvature and therespective centers of curvature of surfaces 242A and 246A are conjugatepoints with respect to beam-splitter 248. Surfaces 242B and 246B areconcave spherical surfaces with nominally the same radii of curvature.The centers of curvature of surfaces 242B and 246B are the same as thecenters of curvature of surfaces 246A and 242A, respectively.

The centers of curvature of the surfaces of concentric lens 250 andplano convex lens 256 are nominally the same as the center of curvatureof surfaces 242B and 246A. The centers of curvature of the surfaces ofconcentric lens 254 and plano convex lens 258 are nominally the same asthe center of curvature of surfaces 242A and 246B. The radii ofcurvature of surfaces 260 and 264 are nominally the same and the radiiof curvature of surfaces 262 and 266 are nominally the same. There maybe a small gap between the convex surface and corresponding concavesurface of lenses 256 and 250, respectively, and there may be acorresponding small gap between the convex surface and correspondingconcave surface of lenses 258 and 254, respectively.

The sagittal field of catadioptric imaging system 210 is a flat fieldand the tangential field is also a flat field for a certain object fieldwhen the Petzval sum is zero, i.e., $\begin{matrix}{{{2{\sum\limits_{j = 1}^{p - 1}\quad{( {\frac{1}{n_{j}} - \frac{1}{n_{j + 1}}} )\frac{1}{r_{j}}}}} + {\frac{1}{n_{p}}\frac{2}{r_{p}}}} = 0} & (7)\end{matrix}$where r_(j) is the radius of curvature of surface j, r_(p) is the radiusof curvature of the mirror surface, and n_(j) is the index of refractionof the media located on the beam incidence side of surface j such asshown diagrammatically in FIG. 2 b. The condition for the generation ofan achromatic anastigmat at wavelength λ_(c) is accordingly given by theequation $\begin{matrix}{{\partial\frac{\lbrack {{2{\sum\limits_{j = 1}^{p - 1}\quad{( {\frac{1}{n_{j}} - \frac{1}{n_{j + 1}}} )\frac{1}{r_{j}}}}} + {\frac{1}{n_{p}}\frac{2}{r_{p}}}} \rbrack}{\partial\lambda}} = 0.} & (8)\end{matrix}$

Two considerations in the selection of the radii of curvature ofsurfaces 242B and 246B and surfaces 162 and 166 are the area of thesystem pupil function of the imaging system 210 and the size of theobject field that can be effectively used with respect to image quality.The first two considerations place competing demands of the selection ofthe radii of curvature of surfaces 242B and 246B and surfaces 162 and166. Third and fourth considerations are with respect to the conditionsset out in Equations (7) and (8). A fifth consideration in the selectionof the media of the lenses of imaging system 210 is the transmissionproperties of the media for the range of wavelengths to be used in anend use application.

For an example of an achromatic anastigmat design for deep UV operation,the media of elements 240, 244, 256, and 258 is selected as CaF₂ and themedia of concentric lenses 252 and 254 is selected as a UV grade fusedsilica or fluorine-doped fused silica (F-SiO₂). Other parameters of theexample achromatic anastigmat design such as the radii of curvature ofsurfaces are listed in Table 1 for λ_(c)=250 nm. With this choice ofmedia, the operation range is down to 170 nm for UV grad fused silicaand down to 155 nm for F-SiO₂. For the achromatic anastigmat designparameters listed in Table 1, the contribution of geometric ray tracingeffects is ≦40 nm for an object field of 1.5 mm in diameter and anumerical aperture NA=0.970 in the object space just outside of theplane surface of plano convex lens 258. TABLE 1 Achromatic AnastigmatDesign for λ_(c) = 250 nm Media j n_(j) r_(j) (mm) CaF₂ 1 1.467297 3.600Fused Silica 2 1.507446 9.256 Vacuum 3 1 18.000 CaF₂ 4 1.467297 50.000

A variant of catadioptric imaging system 210 is shown in FIG. 2 cwherein catadioptric imaging system 100 is an anastigmat that is notachromatic. The media of elements 140 and 144 may comprise CaF₂, BaF₂,or SrF₂ for work down to 140 nm and UV grade fused silica for operationto 180 nm. The respective radii of curvature for anastigmat design atλ=250 nm using CaF₂ are listed in Table 2. For the anastigmat designlisted in Table 2, the contribution of geometric ray tracing effects is≦40 nm for an object field of 1.5 mm and a numerical aperture NA=0.970in the object space just outside of the plane surface of plano convexlens 258. TABLE 2 Anastigmat Design for λ = 250 nm Media j n_(j) r_(j)(mm) CaF₂ 1 1.467297 7.950 Air 2 1 12.000 CaF₂ 3 1.467297 50.000

The respective radii of curvature for anastigmat design at λ=250 nmusing fused silica are listed in Table 3. For the anastigmat designlisted in Table 3, the contribution of geometric ray tracing effects is≦40 nm for an object field of 1.5 mm and a numerical aperture NA=0.970in the object space just outside of the plane surface of plano convexlens 258. TABLE 3 Anastigmat Design for λ = 250 nm Media j n_(j) r_(j)(mm) Fused Silica 1 1.467297 7.950 Air 2 1 12.000 Fused Silica 31.467297 50.000

Intrinsic birefringence of SrF₂ is less than the intrinsic birefringenceof CaF₂ and BaF₂ at 140 nm. However, the intrinsic birefringence of anyone of the three crystalline materials can be accommodated in thecatadioptric imaging system 100 since only an azimuthal section of thelens elements are used and that section can be selected to significantlyreduce the effects of intrinsic birefringence, e.g., with the [111] axisof the crystal aligned parallel to the optic axis of catadioptricimaging system 100 and the [110] axis of the crystal aligned parallel tothe plane of FIG. 2 a.

Another form of anastigmat catadioptric imaging system that may be usedfor catadioptric and catoptric imaging system 100 is the catadioptricimaging system such as described in cited U.S. Provisional PatentApplication No. 60/460,129 (ZI-51) and shown schematically in FIG. 2 f.The catadioptric imaging system shown in FIG. 2 f comprises the sameelements as the astigmatic catadioptric imaging system shown in FIG. 2 cexcept for the omission of element 258. The sagittal field ofcatadioptric imaging system 210 shown in FIG. 2 f is a flat field andthe tangential field is also a flat field for a certain object fieldwhen the Petzval sum is zero, i.e., $\begin{matrix}{{{( {\frac{1}{n_{j}} - \frac{1}{n_{j + 1}}} )\frac{1}{r_{j}}} + {2{\sum\limits_{j = 2}^{p - 1}\quad{( {\frac{1}{n_{j}} - \frac{1}{n_{j + 1}}} )\frac{1}{r_{j}}}}} + {\frac{1}{n_{p}}\frac{2}{r_{p}}}} = 0} & (9)\end{matrix}$where r₁ corresponds to the radius of curvature of surface 260. Anexample of a set of respective radii of curvature for an anastigmatdesign for at λ=250 nm using fused silica comprises the same radii ofcurvature listed in Table 3 for r₂ and r₃ and ½ of the value listed forr₁.

The location of the object plane of catadioptric imaging system 210 isoutside of plano convex lens 158 and on the surface of substrate 60which is shown diagrammatically in FIG. 2 d. The separation of the planesurface of plano convex lens 158 and the surface of substrate 60 is h.The object plane of catadioptric imaging system 210 may also be locatedin the interior of substrate 60 which is shown diagrammatically in FIG.2 e. When used in an end use application where the resolution ofcatadioptric imaging system 210 is restricted to a Δθ such as describedwith respect to Equation (6), the spherical aberration introduced bytransmission through plane surfaces shown in FIGS. 2 d and 2 e generallywill not impact on the performance of the imaging system 210.

An advantage of the catadioptric imaging system 210 is that as aconsequence of the spherical aberration introduced by transmissionthrough plane surfaces, the effective angle of incidence θ₁ can bescanned by introducing a scan in h.

For those end use applications where compensation is required for thespherical aberration introduced by transmission through plane surfaces,procedures may be use such as described in commonly owned U.S.Provisional Application No. 60/444,707 (ZI-44) and U.S. patentapplication Ser. No. 10/771,785 (ZI-44) of which both are entitled“Compensation for Effects of Mismatch in Indices of Refraction at aSubstrate-Medium Interface in Confocal and Interferometric ConfocalMicroscopy,” both are to Henry A. Hill, and the contents of both areherein incorporated in their entirety by reference.

The description of imaging system 100 is continued with reference toFIG. 1 c. Lens sections 40 and 44 are sections, e.g., pie sections, oflens 240 and 244 shown in FIG. 2 a. Lens elements 250, 256, 254, and 258in FIG. 1 c are the same elements lens elements 250, 256, 254, and 258in FIG. 2 a. Convex lens 52 has a center of curvature the same as thecenter of curvature of convex lens 250. Convex lenses 250 and 52 arebonded together with pinhole array beam-splitter 12 in between. Pinholearray beam-splitter 12 is shown in FIG. 1 d. The pattern of pinholes inpinhole array beam-splitter is chosen so that the image of pinhole arraybeam-splitter 12 on detector 70 to match the pixel pattern of detector70. An example of a pattern is a two dimensional array of equally spacedpinholes in two orthogonal directions. The pinholes may comprisecircular apertures, rectangular apertures, or combinations thereof suchas described in commonly owned U.S. patent application Ser. No.09/917,402 (ZI-15) entitled “Multiple-Source Arrays for Confocal andNear-field Microscopy” by Henry A. Hill and Kyle Ferrio of which thecontents thereof are incorporated herein in their entirety by reference.The pinholes may also comprise microgratings such as described in citedU.S. Provisional Patent Application No. 60/459,425 and U.S. patentapplication Ser. No. 10/816,180, (ZI-50) filed Apr. 1, 2004 and entitled“Apparatus and Method for Joint Measurement Of Fields OfScattered/Reflected Or Transmitted Orthogonally Polarized Beams By AnObject In Interferometry”. A nonlimiting example of a pinhole array forpinhole array beam-splitter 12 is shown in FIG. 1 d having a spacingbetween pinholes of b with aperture size a.

With reference to FIGS. 1 b and 1 c, a first portion of input beam 24 isreflected by non-polarizing beam-splitter 54A and incident on pinholearray beam-splitter 12 after reflection by mirrors 54B and 54C. Theangle of incidence of the reference beam at pinhole array beam-splitter12 is selected to correspond to θ_(D) (see Equation (3) and relateddiscussion) in the plane of FIG. 1 c. The reference beam mayalternatively be incident on pinhole array beam-splitter 12 from theopposite side with an angle of incidence equal to θ_(D) in the plane ofFIG. 1 c. The direction of propagation of the reference beam incidentfrom the opposite side is the mirror image about the surface of pinholearray beam-splitter 12 of the direction of propagation of the referencebeam shown in FIG. 1 c. A portion of the beam reflected by beam-splitter54A and incident on pinhole array beam-splitter 12 is reflected asreference beam components of output beam 32A and 32B (see FIG. 1 b).

A general property of various embodiments of the present invention isthat the integrated interference cross-term between the reference beamand the respective return measurement beam across the spot by thedetector 70 will have a peak in detection sensitivity for thereflected/scattered return measurement beam components of beams 28C thathave an angle of diffraction corresponding to θ_(D). By the selection ofthe angle of incidence of the reference beam at pinhole arraybeam-splitter 12 to correspond to θ_(D), the reflected/scattered returnmeasurement beam components of beams 28C across the spot onbeam-splitter 12 remains in phase with the reference beam. As aconsequence, the interference cross-term between the reference beam andthe respective return measurement beam across the spot comprises aconstant term and an oscillatory term. In this case, the interferencecross-term integrates across the spot to a meaningful non-zero valueproportional to the constant term. Whereas the projection of the spatialwavelength onto beam splitter 12 for the other diffraction angles of thereturn measurement beam is not equal to the projection of the wavelengthof the reference beam onto beam-splitter 12 so that the returnmeasurement beam components of beams 28C across the spot onbeam-splitter 12 do not remain in phase with the reference beam.Accordingly, the interference cross-term for those diffractiondirections oscillates across the spot comprising two oscillatory termsand therefore their contributions integrate to zero or reduced values.

Another general property of various embodiments of the present inventionis that the since the reference beam is generated with the angle ofincidence at pinhole array beam-splitter 12 selected to correspond toθ_(D), the phases of conjugated quadratures corresponding to theinterference cross-term between the reference beam and thereflected/scattered return measurement beam components of beams 28C inthe electrical interference signal 72 generated by detection of mixedoutput beams by detector 70 is ζ_(x) and ζ_(y) (see Equations (1) and(2) and associated description) with no dependences on either x or y.This is an important feature since the phase represented in conjugatedquadratures is a function only of the reflecting properties and locationof the defect and/or artifact in addition to a fixed offset error in theinterferometric metrology system. A corollary statement is that theaccuracy to which the location of a defect and/or artifact on or insubstrate being imaged can be measured is not affected by displacementsof a pinhole corresponding to a detector or of a detector pixel used inmeasuring the respective conjugated quadratures other than contributingto a phase redundancy mod 2π.

A second portion of beam 24 is transmitted by beam-splitter 54A asmeasurement beam 24A after reflection by mirror 54D. Measurement beam24A is incident on substrate 60 and a portion thereof is reflectedand/or scattered as return measurement beam components of beams 28C. Theangle of incidence of the measurement beam at substrate 60 is selectedto correspond to θ₁ (see Equation (3) and related discussion). Returnmeasurement beam components of beam 28C are imaged by catadioptricimaging system 100 to pinhole array beam-splitter 12 and a portionthereof is transmitted as return measurement beam components of outputbeams 32A and 32B.

The next step is the imaging of output beams 32A and 32B by imagingsystem 110 to an array of spots that coincide with the pixels of amulti-pixel detector such as a CCD to generate an array of electricalinterference signals 72. The array of electrical interference signals istransmitted to signal processor and controller 80 for subsequentprocessing.

Conjugated quadratures of fields of the return measurement beam areobtained by single-homodyne detection in the first embodiment of thepresent invention. For the single-homodyne detection, a set of fourmeasurements of electrical interference signals 72 is made. For each ofthe four measurements of the electrical interference signals 72, a knownphase shift is introduced between the reference beam component andrespective return measurement beam component of output beams 32A and32B. A nonlimiting example of a known set of phase shifts comprise 0,π/4, π/2, and 3π/2 radians.

Input beam 24 comprises in the first embodiment one frequency component.The phase shifts are generated in the first embodiment by shifting thefrequency of the input beam 24 between known frequency values. There isa difference in optical path length between the reference beamcomponents and the respective return beam components of output beams 32Aand 32B and as a consequence, a change in frequency of input beam 24will generate a corresponding phase shift between the reference beamcomponents and the respective return beam components of output beams 32Aand 32B.

For an optical path difference L between the reference beam componentsand the respective return measurement beam components of output beams32A and 32B, there will be for a frequency shift Δf a correspondingphase shift φ where $\begin{matrix}{\varphi = {2\quad\pi\quad{L( \frac{\Delta\quad f}{c} )}}} & (10)\end{matrix}$and c is the free space speed of light. Note that L is not a physicalpath length difference and depends for example on the average index ofrefraction of the measurement beam and the return measurement beampaths. For an example of a phase shift φ=π, 3π, 5π, . . . and a value ofL=0.25 m, the corresponding frequency shift Δf=600 MHz, 1.8 GHz, 3.0GHz, . . . .

The frequency of input beam 24 is determined by beam-conditioner 22according to control signal 74 generated by electronic processor andcontroller 80. Source 18 of input beam 20, such as a laser, can be anyof a variety of single frequency lasers.

Two different modes of operation are described for the acquisition ofthe four electrical interference signal values. The first mode to bedescribed is a step and stare mode wherein substrate 60 is steppedbetween fixed locations for which image information is desired. Thesecond mode is a scanning mode. In the step and stare mode forgenerating a one-, a two-, or a three-dimensional image of substrate 60,substrate 60 is translated by stage 90 wherein substrate 60 is mountedon wafer chuck 84 with wafer chuck 84 mounted on stage 90. The positionof stage 90 is controlled by transducer 82 according to servo controlsignal 78 from electronic processor and controller 80. The position ofstage 90 is measured with respect to a reference system by metrologysystem 88 and position information acquired by metrology system 88 istransmitted to electronic processor and controller 80 to generate anerror signal for use in the position control of stage 90. The referencesystem may correspond to the reference frame of a lithography tool usedfor writing to a wafer or an inspection tool for lithography masks orreticles. Metrology system 88 may comprise for example lineardisplacement and angular displacement interferometers and cap gauges.

Electronic processor and controller 80 translates stage 90 to a desiredposition and then acquires the set of four electrical interferencesignal values corresponding to the set of four phase shifts 0, π/4, π/2,and 3π/2. After the acquisition of the sequence of four electricalinterference signal values, electronic processor and controller 80repeats the procedure for the next desired position of stage 90. Theelevation and angular orientation of substrate 60 is controlled bytransducers 86A and 86B.

The second of the two modes for the acquisition of the electricalinterference signal values is next described wherein the electricalinterference signal values are obtained with the position of stage 90scanned in one or more directions. In the scanning mode, source 18 ispulsed at times controlled by signal 92 from signal processor andcontroller 80. Source 18 is pulsed at times corresponding to theregistration of the conjugate image of pinholes of pinhole arraybeam-splitter 12 with positions on and/or in substrate 60 for whichimage information is desired.

There will be a restriction on the duration or “pulse width” of a beampulse τ_(p1) produced by source 18 as a result of the continuousscanning used in the scanning mode of the first embodiment. Pulse widthτ_(p1) will be a parameter that in part controls the limiting value forspatial resolution in the direction of a scan to a lower bound ofτ_(p1)v,  (11)where v is the scan speed. For example, with a value of τ_(p1)=50 nsecand a scan speed of v=0.20 m/sec, the limiting value of the spatialresolution τ_(p1)v in the direction of scan will beτ_(p1)v=10 nm.  (12)

The frequency of input beam 24 is controlled by signal 74 fromelectronic processor and controller 80 to correspond to a frequency of aset of four frequencies that will yield the desired phase shift of theset of four phase shifts between the reference and return measurementbeam components of output beams 32A and 32B. In the first mode for theacquisition of the electrical interference signal values, each of thesets of four electrical interference signal values corresponding to theset of four phase shift values are generated by a single pixel ofdetector 70. In the second mode for the acquisition of the electricalinterference signal values, the set of four electrical interferencesignal values corresponding to the set of four phase shift values aregenerated by a conjugate set of four different pixels of detector 70.Thus in the second mode of acquisition, the differences in pixelefficiency and the differences in sizes of pinholes in pinhole arraybeam-splitter 12 need to be compensated in the signal processing byelectronic processor and controller 80 to obtain conjugated quadraturesof fields of return measurement beam components.

The advantage of the second mode is that the electrical interferencesignal values are acquired in a scanning mode which increases throughput of the interferometric confocal microscopy system of the firstembodiment.

The description beam-conditioner 22 is the same as the description givenfor the two frequency generator and frequency-shifter described in citedU.S. Provisional Application No 60/442,858 (ZI-47) and U.S. patentapplication Ser. No. 10/765,369, filed Jan. 27, 2004 (ZI-47) andentitled “Apparatus and Method for Joint Measurements of ConjugatedQuadratures of Fields of Reflected/Scattered Beams by an Object inInterferometry.”

Reference is made to FIG. 1 e where beam-conditioner 22 is firstdescribed generally as a two-frequency generator and afrequency-shifter. Beam-conditioner 22 may be operated to generate abeam 24 that has either a single frequency-shifted component or twofrequency-shifted components. Beam-conditioner 22 is operated in thesingle frequency-shifted mode for the first embodiment.

Beam-conditioner 22 comprises acousto-optic modulators 1120, 1126, 1130,1132, 1142, 1146, 1150, 1154, 1058, and 1062; beam-splitter 1168; andmirror 1166. Input beam 20 is incident on acousto-optic modulator 1120with a plane of polarization parallel to the plane of FIG. 1 e. A firstportion of beam 20 is diffracted by acousto-optic modulator 1120 as beam1122 and then by acousto-optic modulator 1126 as beam 1128 having apolarization parallel to the plane of FIG. 1 e. A second portion of beam20 is transmitted as a non-diffracted beam 1124 having a plane ofpolarization parallel to the plane of FIG. 1 e.

For beam-conditioner 22 operated to generate the singlefrequency-shifted component for beam 24, the acoustic power toacousto-optic modulator 1120 is switched between two states. One stateis the off state where the amplitude of diffracted beam 1122 in zero andin the on state, the amplitude of non-diffracted beam 1124 is nominallyzero. The on or off states of acousto-optic modulator 1120 is controlledby signal 74 generated by electronic processor and controller 80

Acousto-optic modulators 1120 and 1126 may be of either thenon-isotropic Bragg diffraction type or of the isotropic Braggdiffraction type. The frequency shifts introduced by acousto-opticmodulators 1120 and 1126 are of the same sign and equal to ½ of afrequency shift Δf that will generate in interferometer 10 a π/2 phasedifference between a reference beam and a measurement beam that have adifference in frequency equal to frequency shift Δf. The direction ofpropagation of beam 1128 is parallel to the direction of propagation ofbeam 1124.

Continuing with FIG. 1 e, beam 1128 is incident on acousto-opticmodulator 1132 and is either diffracted by acousto-optic modulator 1132as beam 1134 or transmitted by acousto-optic modulator 1132 as beam 1136according to control signal 74 from electronic processor and controller80. When beam 1134 is generated, beam 1134 is diffracted byacousto-optic modulators 1142, 1146, and 1150 as a frequency shiftedbeam component of beam 1152. The frequency shifts introduced byacousto-optic modulators 1132, 1142, 1146, and 1150 are all in the samedirection and equal in magnitude to Δf/2. Thus the net frequency shiftintroduced by acousto-optic modulators 1132, 1142, 1146, and 1150 is±2Δf. The net frequency shift introduced by acousto-optic modulators1120, 1126, 1132, 1142, 1146, and 1150 is Δf±2Δf that in turn willgenerate a respective relative phase shift of π/2±π between therespective reference and measurement beams in interferometer 10.

When beam 1136 is generated, beam 1136 is transmitted by acousto-opticmodulator 1150 according to control signal 74 from electronic processorand controller 80 as a non-frequency shifted beam component of beam1152. The frequency shift introduced by acousto-optic modulators 1120,1126, 1132, and 1150 is Δf which will generate a respective relativephase shift of π/2 between the respective reference and measurementbeams in interferometer 10.

Beam 1124 is incident on acousto-optic modulator 1130 and is eitherdiffracted by acousto-optic modulator 1130 as beam 1140 or transmittedby acousto-optic modulator 1130 as beam 1138 according to control signal74 from electronic processor and controller 80. When beam 1140 isgenerated, beam 1140 is diffracted by acousto-optic modulators 1154,1158, and 1162 as a frequency-shifted beam component of beam 1164. Thefrequency shifts introduced by acousto-optic modulators 1130, 1154,1158, and 1162 are all in the same direction and equal to ±Δf/2. Thusthe net frequency shift introduced by acousto-optic modulators 1130,1154, 1158, and 1162 is ±2Δf and will generate a relative phase shift of±π between the respective reference and measurement beams ininterferometer 10. The net frequency shift introduced by acousto-opticmodulators 1120, 1130, 1154, 1158, and 1162 is ±2Δf and will generate arespective relative phase shift of ±π between the respective referenceand measurement beams in interferometer 10.

When beam 1138 is generated, beam 1138 is transmitted as a non-frequencyshifted beam component of beam 1164. The corresponding frequency shiftintroduced by acousto-optic modulators 1120, 1130, and 1162 is 0 andwill generate a respective relative phase shift of 0 between therespective reference and measurement beams in interferometer 10.

Beams 1152 and 1164 may be used directly as input beam 24 when anembodiment requires spatially separated reference and measurement beamsfor an input beam. When an embodiment of the present invention requirescoextensive reference and measurement beams as an input beam, beam 1152and 1164 are next combined by beam-splitter 1168 to form beam 24.Acousto-optic modulators 1120, 1126, 1130, 1132, 1142, 1146, 1150, 1154,1158, and 1162 may be either of the non-isotropic Bragg diffraction typeor of the isotropic Bragg diffraction type. Beams 1152 and 1164 are bothpolarized in the plane of FIG. 1 e for either non-isotropic Braggdiffraction type or of the isotropic Bragg diffraction type andbeam-splitter 1168 is of the non-polarizing type.

In the first embodiment of the present invention, the processing of themeasured arrays of sets of four measured electrical interference signalvalues for the determination of conjugated quadratures of fields ofreturn measurement beams is described herein as a special case of thebi-homodyne detection method and is also described for example in citedU.S. Pat. No. 6,445,453 (ZI-14).

Referring to the bi-homodyne detection method such as described in citedU.S Provisional Patent Application No. 60/442,858 (ZI-47) and cited U.S.patent application Ser. No. 10/765,369, filed Jan. 27, 2004 (ZI-47)entitled “Apparatus and Method for Joint Measurements of ConjugatedQuadratures of Fields of Reflected/Scattered and Transmitted Beams by anObject in Interferometry” wherein conjugated quadratures are obtainedjointly, a set of four electrical interference signal values is obtainedfor each spot on and/or in substrate 60 being imaged. The set of fourelectrical interference signal values S_(j), j=1, 2, 3, 4 used forobtaining conjugated quadratures of fields for a single a spot on and/orin a substrate being imaged is represented for the bi-homodyne detectionwithin a scale factor by the formula $\begin{matrix}{S_{j} = {P_{j}{\sum\limits_{m = 1}^{2}\begin{Bmatrix}{{\xi_{j}^{2}{A_{m}}^{2}} + {\zeta_{j}^{2}{B_{m}}^{2}} + {\eta_{j}^{2}{C_{m}}^{2}} +} \\{{{{\zeta_{j}\eta_{j}2{B_{m}}C_{m}}}\cos\quad\varphi\quad B_{m}C_{m}ɛ_{m,j}} +} \\{{{{\xi_{j}\zeta_{j}2{A_{m}}B_{m}}}\cos\quad\varphi\quad A_{m}B_{m}ɛ_{m,j}} +} \\{{{{ɛ_{m,j}\xi_{j}{\eta_{j}\lbrack {1 - ( {- 1} )^{m}} \rbrack}{A_{m}}C_{m}}}\cos\quad\varphi_{A_{m}C_{m}}} +} \\{{{ɛ_{m,j}\xi_{j}{\eta_{j}\lbrack {1 + ( {- 1} )^{m}} \rbrack}{A_{m}}C_{m}}}\sin\quad\varphi_{A_{m}C_{m}}}\end{Bmatrix}}}} & (13)\end{matrix}$where coefficient A_(m) represents the amplitude of the reference beamcorresponding to the frequency component of the input beam 24 that hasindex m; coefficient B_(m) represents the amplitude of the backgroundbeam corresponding to reference beam A_(m); coefficient C_(m) representsthe amplitude of the return measurement beam corresponding to referencebeam A_(m); P_(j) represents the integrated intensity of the firstfrequency component of the input beam 24 pulse j of a sequence of 4pulses; and an example set of values for ε_(m,j) are listed in Table 4.There are other set of values for ε_(m,j) that may be used inembodiments of the present invention wherein the other set of values forε_(m,j) satisfy the conditions set out in subsequent Equations (14) and(15) herein.

The change in the values of ε_(m,j) from 1 to −1 or from −1 to 1corresponds to changes in relative phases of respective reference andmeasurement beams. The coefficients ξ_(j), ζ_(j), and η_(j) representeffects of variations in properties of a conjugate set of four pinholessuch as size and shape if used in the generation of the spot on and/orin substrate 60 and the sensitivities of a conjugate set of fourdetector pixels corresponding to the spot on and/or in substrate 60 forthe reference, background, and the return measurement beam,respectively. In the single-frequency single-homodyne detectionoperating in a non-scanning mode, the conjugate set of pinholescorresponds to a single pinhole and the conjugate set of four pixelscorresponds to a single pixel. The conjugate set of four pinholescomprise pinholes of pinhole array beam-splitter 12 that are conjugateto a spot in or on the substrate being imaged at different times duringthe scan. TABLE 4 ε_(m,j) m j 1 2 1 1 1 2 1 −1 3 −1 −1 4 −1 1

The relationships cos φ_(A) ₂ _(C) ₂ =sin φ_(A) ₁ _(C) ₁ and cos φ_(A) ₄_(C) ₄ =sin φ_(A) ₃ _(C) ₃ have been used in deriving Equation (13)without departing from either the scope or spirit of the presentinvention since cos φ_(A) ₂ _(C) ₂ =±sin φ_(A) ₁ _(C) ₁ and cos φ_(A) ₄_(C) ₄ =±sin φ_(A) ₃ _(C) ₃ by control of the relative phase shiftsbetween corresponding reference and return measurement beam componentsin beam 32.

It has also been assumed in Equation (13) that the ratios |A₂|/|A₁| and|A₄|/|A₃| are not dependent on j or on the value of P_(j). In order tosimplify the representation of S_(j) so as to project the importantfeatures without departing from either the scope or spirit of thepresent invention, it is also assumed in Equation (13) that thecorresponding ratios of the amplitudes of the return measurement beamsare not dependent on j or on the value of P_(j). However, the ratios|C₂|/|C₁| and |C₄|/|C₃|will be different from the ratio |A₂|/|A₁| and|A₄|/|A₃|, respectively, the ratios of the amplitudes of the measurementbeam components corresponding to A₂ and A₁ are different from the ratio|A₂|/|A₁| and corresponding to A₄ and A₃ are different from the ratio|A₄|/|A₃|.

The change in phases φ_(A) _(m) _(B) _(m) _(ε) _(m,j) for a change inε_(m,j) may be different from π for embodiments where phase shifts areintroduced between the arrays of reference and measurement beams bychanging the frequency of an input beam component. It may be of value inevaluating the effects of the background beams to note that the factorcos φ_(B) _(m) _(C) _(m) _(ε) _(m,j) may be written ascos [φ_(A_(m)C)_(m) + (φ_(B_(m)C_(m)ɛ_(m, j)) − φ_(A_(m)C_(m)))]where the phase difference (φ_(B) _(m) _(C) _(m) _(ε) _(m,j) −φ_(A) _(m)_(C) _(m) ) is the same as the measured phase φ_(A) _(m) _(B) _(m) _(ε)_(m,j) .

It is evident from inspection of Equation (13) that the components ofconjugated quadratures ε_(m,j)|C_(m)|cos φ_(A) _(m) _(C) _(m) andε_(m,j)|C_(m)|sin φ_(A) _(m) _(C) _(m) in Equation (13) are functionsthat have mean values of zero since $\begin{matrix}{{{\sum\limits_{j = 1}^{4}ɛ_{m,j}} = 0},\quad{m = 1},2.} & (14)\end{matrix}$Another important property is that the conjugated quadraturesε_(m,j)|C_(m)|cos φ_(A) _(m) _(C) _(m) and ε_(m′,j)|C_(m′)|sin φ_(A)_(m′) _(C) _(m′) , are orthogonal over the range of m=1,2 for m≠m′ sinceε_(m,j) and ε_(m′,j) are orthogonal over the range of j=1, 2, 3, 4,i.e., $\begin{matrix}{{{\sum\limits_{j = 1}^{4}\quad{ɛ_{m,j}ɛ_{m,j}^{\prime}}} = {4\delta_{m,m}^{\prime}}},} & (15)\end{matrix}$where δ_(m,m′) is the Kronecker delta defined byδ_(m,m′)=1 for m=m′,δ_(m,m′)=0 for m≠m′.  (16)

Information about conjugated quadratures |C_(m)|cos φ_(A) _(m) _(C) _(m)and |C_(m)|sin φ_(A) _(m) _(C) _(m) is obtained using a digital filterF_(m)(S_(j)) on signals S_(j) that are based on the orthogonalityproperties of the ε_(m,j) as represented by Equation (15). Thedefinition of F_(m)(S_(j)) and the output of digital filter F(S_(j)) are$\begin{matrix}{{{{{{{F_{m}( S_{j} )} = {{\sum\limits_{j = 1}^{4}\quad{ɛ_{m,j}\frac{S_{j}}{P_{j}^{\prime}\xi_{j}^{\prime\quad 2}}}} = {{\sum\limits_{m^{\prime} = 1}^{2}{{A_{m}^{\prime}}^{2}{\sum\limits_{j = 1}^{4}\quad{{ɛ_{m,j}( \frac{P_{j}}{P_{j}^{\prime}} )}( \frac{\xi_{j}^{2}}{\xi_{j}^{\prime\quad 2}} )}}}} + {\sum\limits_{m^{\prime} = 1}^{2}{{B_{m}^{\prime}}^{2}{\sum\limits_{j = 1}^{4}\quad{{ɛ_{m,j}( \frac{P_{j}}{P_{j}^{\prime}} )}( \frac{\zeta_{j}^{2}}{\xi_{j}^{\prime\quad 2}} )}}}} + {\sum\limits_{m^{\prime} = 1}^{2}{{C_{m}^{\prime}}^{2}{\sum\limits_{j = 1}^{4}\quad{{ɛ_{m,j}( \frac{P_{j}}{P_{j}^{\prime}} )}( \frac{\eta_{j}^{2}}{\xi_{j}^{\prime\quad 2}} )}}}} + \quad{\lbrack {1 - ( {- 1} )^{m}} \rbrack{A_{m}}C_{m}}}}}}\cos\quad\varphi_{A_{m}C_{m}}\quad{\sum\limits_{j = 1}^{4}\quad{( \frac{P_{j}}{P_{j}^{\prime}} )( \frac{\xi_{j}\eta_{j}}{\xi_{j}^{\prime\quad 2}} ){\sum\limits_{m^{\prime} = 1}^{2}\quad{ɛ_{m,j}\quad ɛ_{m,j}^{\prime}}}}}} + {\lbrack {1 + ( {- 1} )^{m}} \rbrack{{{A_{m}{C_{m}}\sin\quad\varphi_{A_{m}C_{m}}{\sum\limits_{j = 1}^{4}\quad{( \frac{P_{j}}{P_{j}^{\prime}} )( \frac{\xi_{j}\eta_{j}}{\xi_{j}^{\prime\quad 2}} ){\sum\limits_{m^{\prime} = 1}^{2}\quad{ɛ_{m,j}\quad ɛ_{m,j}^{\prime}}}}}} + {2{\sum\limits_{m^{\prime} = 1}^{2}{{A_{m}^{\prime}}B_{m}^{\prime}}}}}}{\sum\limits_{j = 1}^{4}\quad{{ɛ_{m,j}( \frac{P_{j}}{P_{j}^{\prime}} )}( \frac{\xi_{j}\zeta_{j}}{\xi_{j}^{\prime\quad 2}} )\cos\quad\varphi_{A_{m}^{\prime}B_{m}^{\prime}ɛ_{m,j}^{\prime}}}}} + {2{\sum\limits_{m^{\prime} = 1}^{2}{{B_{m}^{\prime}}C_{m}^{\prime}}}}}}{\sum\limits_{j = 1}^{4}\quad{{ɛ_{m,j}( \frac{P_{j}}{P_{j}^{\prime}} )}( \frac{\zeta_{j}\eta_{j}}{\xi_{j}^{\prime\quad 2}} )\cos\quad{\varphi_{B_{m}^{\prime}C_{m}^{\prime}ɛ_{m,j}^{\prime}}.}}}} & (17)\end{matrix}$where ξ′_(j) and P′_(j) are values used in the digital filters torepresent ξ_(j) and P_(j), respectively.

The conjugated quadratures |C_(m)|cos φ_(A) _(m) _(C) _(m) and|C_(m)|sin φ_(A) _(m) _(C) _(m) correspond to the interferencecross-term between the reference beam and the reflected/scattered returnmeasurement beam components of beams 28C in the electrical interferencesignal 72 generated by detection of mixed output beams by detector 70.The phase of the conjugated quadratures |C_(m)|cos φ_(A) _(m) _(C) _(m)and |C_(m)|sin φ_(A) _(m) _(C) _(m) , i.e., φ_(A) _(m) _(C) _(m) ,relates to ζ_(x) and ζ_(y) (see Equations (1) and (2) and associateddescription).

The parameters $\begin{matrix}{\lbrack {( \frac{A_{2}}{A_{1}} )( \frac{C_{2}}{C_{1}} )} \rbrack,} & (18) \\\lbrack {( \frac{A_{4}}{A_{3}} )( \frac{C_{4}}{C_{3}} )} \rbrack & (19)\end{matrix}$need to be determined in order complete the determination of aconjugated quadratures. The parameters given in Equations (18) and (19)can be measured for example by introducing π/2 phase shifts into therelative phase of the reference beam and the measurement beam andrepeating the measurement for the conjugated quadratures. The ratios ofthe amplitudes of the conjugated quadratures corresponding to (sin φ_(A)₁ _(C) ₁ /cos φ_(A) ₁ _(C) ₁ ) and (sin φ_(A) ₃ _(C) ₃ /cos φ_(A) ₃ _(C)₃ ) from the first measurement divided by the ratios of the amplitudesof the conjugated quadratures corresponding to (sin φ_(A) ₁ _(C) ₁ /cosφ_(A) ₁ _(C) ₁ ) and (sin φ_(A) ₃ _(C) ₃ /cos φ_(A) ₃ _(C) ₃ )respectively, from the second measurement are equal to $\begin{matrix}{\lbrack {( \frac{A_{2}}{A_{1}} )( \frac{C_{2}}{C_{1}} )} \rbrack^{2},} & (20) \\{\lbrack {( \frac{A_{4}}{A_{3}} )( \frac{C_{4}}{C_{3}} )} \rbrack^{2},} & (21)\end{matrix}$respectively.

Note that certain of the factors in Equation (17) have nominal values of4 within a scale factor, e.g., have nominal values of either 0 or 4,e.g., $\begin{matrix}{{\sum\limits_{j = 1}^{4}\quad{( \frac{P_{j}}{P_{j}^{\prime}} )( \frac{\xi_{j}\eta_{j}}{\xi_{j}^{\prime\quad 2}} ){\sum\limits_{m^{\prime} = 1}^{2}\quad{ɛ_{m,j}\quad ɛ_{m,j}^{\prime}}}}} \cong {\sum\limits_{j = 1}^{4}\quad{( \frac{P_{j}}{P_{j}^{\prime}} )( \frac{\xi_{j}\eta_{j}}{\xi_{j}^{\prime\quad 2}} )\delta_{m,m}^{\prime}}} \cong {4\delta_{m,m}^{\prime}}} & (22)\end{matrix}$where δ_(m,m)′ is the Kronecker delta defined by Equation (16). Thescale factors corresponds to the average value for the ratio of(ξ′_(j))²/(ξ_(j)η_(j)) assuming that the average values ofP_(j)/P′_(j)≅1.

Certain other of the factors in Equation (17) have nominal values ofzero, e.g., $\begin{matrix}{{{\sum\limits_{j = 1}^{4}\quad{{ɛ_{m,j}( \frac{P_{j}}{P_{j}^{\prime}} )}( \frac{\xi_{j}^{2}}{\xi_{j}^{\prime\quad 2}} )}} \cong 0},{{\sum\limits_{j = 1}^{4}\quad{{ɛ_{m,j}( \frac{P_{j}}{P_{j}^{\prime}} )}( \frac{\zeta_{j}^{2}}{\xi_{j}^{\prime\quad 2}} )}} \cong 0},{{\sum\limits_{j = 1}^{4}\quad{{ɛ_{m,j}( \frac{P_{j}}{P_{j}^{\prime}} )}( \frac{\eta_{j}^{2}}{\xi_{j}^{\prime\quad 2}} )}} \cong 0.}} & (23)\end{matrix}$The remaining factors, $\begin{matrix}{{\sum\limits_{j = 1}^{4}\quad{{ɛ_{m,j}( \frac{P_{j}}{P_{j}^{\prime}} )}( \frac{\xi_{j}\zeta_{j}}{\xi_{j}^{\prime\quad 2}} )\cos\quad\varphi_{A_{m}^{\prime}B_{m}^{\prime}ɛ_{m,j}^{\prime}}}},{\sum\limits_{j = 1}^{4}\quad{{ɛ_{m,j}( \frac{P_{j}}{P_{j}^{\prime}} )}( \frac{\zeta_{j}\eta_{j}}{\xi_{j}^{\prime\quad 2}} )\cos\quad\varphi_{B_{m}^{\prime}C_{m}^{\prime}ɛ_{m,j}^{\prime}}}}} & (24)\end{matrix}$will have nominal magnitudes ranging from of approximately zero toapproximately 4 times a cosine factor and either the average value offactor (P_(j)/P′_(j))(ξ_(j)ζ_(j)/ξ′_(j) ²) or(P_(j)/P′_(j))(ζ_(j)η_(j)/ξ′_(j) ²) depending on the propertiesrespective phases. For portion of the background with phases that do nottrack to a first approximation the phases of the respective measurementbeams, the magnitudes of all of the terms listed in the Equation (24)will be approximately zero. For the portion of the background withphases that do track to a first approximation the phases of therespective measurement beams, the magnitudes of the terms listed inEquation (24) will be approximately 4 times a cosine factor and eitherthe average value of factor (P_(j)/P′_(j))(ξ_(j)ζ_(j)/ξ′_(j) ²) or(P_(j)/P′_(j))(ζ_(j)η_(j)/ξ′_(j) ²).

The two potentially largest terms in Equations (17) are generally theterms that have the factors$\sum\limits_{m^{\prime} = 1}^{4}{{A_{m}^{\prime}}^{2}{\quad\quad}{and}\quad{\sum\limits_{m^{\prime} = 1}^{4}{{B_{m}^{\prime}}^{2}.}}}$However, the corresponding terms are substantially eliminated inembodiments using the bi-homodyne detection method as result of theproperties of the factors listed in Equation (23).

The largest contribution from effects of background is represented bythe contribution to the interference term between the reference beam andthe portion of the background beam generated by the measurement beam30A. This portion of the effect of the background can be measured inembodiments of the bi-homodyne detection method by measuring thecorresponding conjugated quadratures of the portion of the backgroundwith the return measurement beam component of beam 32 set equal to zero,i.e., measuring the respective electrical interference signals S_(j)with substrate 60 removed and with either |A₂|=0 or |A₁|=0 and visaversa and with either |A₄|=0 or |A₃|=0 and visa versa. The measuredconjugated quadratures of the portion of the effect of the backgroundcan than be used to compensate for the respective background effectsbeneficially in an end use application if required.

Information about the largest contribution from effects of backgroundamplitude ξ_(j),ζ_(j)2A_(m)B_(m) and phase φ_(A) _(m) _(B) _(m) _(ε)_(m,j) i.e., the interference term between the reference beam and theportion of background beam generated by the measurement beam 30A, may beobtained by measuring S_(j) for j=1, 2, . . . , 4 as a function ofrelative phase shift between reference beam and the measurement beam 30Awith substrate 60 removed and A_(p)=0, p≠m, and Fourier analyzing themeasured values of S_(j). Such information can be used to help identifythe origin of the respective background.

Other techniques may be incorporated into embodiments of the presentinvention to reduce and/or compensate for the effects of backgroundbeams without departing from either the scope or spirit of the presentinvention such as described in commonly owned U.S. Pat. No. 5,760,901entitled “Method And Apparatus For Confocal Interference Microscopy WithBackground Amplitude Reduction and Compensation,” U.S. Pat. No.5,915,048 entitled “Method and Apparatus for Discrimination In-FocusImages from Out-of-Focus Light Signals from Background and ForegroundLight Sources,” and U.S. Pat. No. 6,480,285 B1 wherein each of the threepatents are by Henry A. Hill. The contents of each of the three citedpatents are herein incorporated in their entirety by reference.

The selection of values for ξ′_(j) is based on information aboutcoefficients ξ_(j) for j=1, 2, . . . , 4 that may be obtained bymeasuring the S_(j) for j=1, 2, . . . , 4 with only the reference beampresent in the interferometer system. In certain embodiments of thepresent invention, this may correspond simply blocking the measurementbeam components of input beam 24 and in certain other embodiments, thismay correspond to simply measuring the S_(j) for j=1, 2, . . . , 4 withsubstrate 60 removed. A test of the correctness of a set of values forξ′_(j) is the degree to which the$\sum\limits_{m^{\prime} = 1}^{2}{A_{m}^{\prime}}^{2}$term in Equation (17) is zero.

Information about coefficients ξ_(j)η_(j) for j=1, 2, . . . , 4 may beobtained for example by scanning an artifact past the respective eightconjugate spots corresponding to the respective eight conjugate detectorpixels with one of the A_(p)≠0 and the remaining A_(p)=0 for p=1, 2 andmeasuring the conjugated quadratures component 2|A_(p)||C_(p)|cos φ_(A)_(p) _(C) _(p) or 2|A_(p)||C_(p)|sin φ_(A) _(p) _(C) _(p) ,respectively. A change in the amplitude of the 2|A_(p)||C_(p)|cos φ_(A)_(p) _(C) _(p) or 2|A_(p)||C_(p)|sin φ_(A) _(p) _(C) _(p) termcorresponds to a variation in ξ_(j)η_(j) as a function of j. Informationabout the coefficients ξ_(j)η_(j) for j=1, 2, . . . , 4 may be used forexample to monitor the stability of one or more elements ofinterferometer system 10.

The bi-homodyne detection method is a robust technique for thedetermination of conjugated quadratures of fields. First, the conjugatedquadratures amplitudes |C₁|cos φ_(A) ₁ _(C) ₁ and |C₁|sin φ_(A) ₁ _(C) ₁are the primary terms in the digitally filtered values F₁ (S) and F₂ (S)as expressed by Equations (17) since as noted in the discussion withrespect to Equation (23), the terms with the factors$\sum\limits_{m^{\prime} = 1}^{2}{{A_{m}^{\prime}}^{2}\quad{and}\quad{\sum\limits_{m^{\prime} = 1}^{2}{B_{m}^{\prime}}^{2}}}$are substantially zero.

Secondly, the coefficients of |C_(m)|cos φ_(A) _(m) _(C) _(m) and|C_(m)|sin φ_(A) _(m) _(C) _(m) terms in Equations (17) are identical.Thus highly accurate measurements of the interference terms between thereturn measurement beam and the reference beam with respect toamplitudes and phases, i.e., highly accurate measurements of conjugatedquadratures of fields can be measured wherein first order variations inξ_(j) and first order errors in normalizations such as (P_(j)/P′_(j))and (ξ_(j) ²/ξ′_(j) ²) enter in only second or higher order. Thisproperty translates in a significant advantage. Also, the contributionsto each component of the conjugated quadratures |C_(m)|cos φ_(A) _(m)_(C) _(m) and |C_(m)|sin φ_(A) _(m) _(C) _(m) from a respective set offour electrical interference signal values have the same window functionand thus are obtained as jointly determined values.

Another distinguishing feature of the bi-homodyne technique is evidentin Equation (17): the coefficients of reference intensity terms |A_(m)|²can be made to be substantially zero by the selection of values forξ′_(j).

It is also evident that since the conjugated quadratures of fields areobtained jointly when using the bi-homodyne detection method, there is asignificant reduction in the potential for an error in tracking phase asa result of a phase redundancy unlike the situation possible insingle-homodyne detection of conjugated quadratures of fields.

The description of processing used in the single-homodyne detection ofthe first embodiment is the same as the description given for thebi-homodyne detection with either of the amplitudes A₂ or A₁ set equalto zero.

For the first embodiment of the present invention, the resolutionrepresented by the high spatial frequency of the measured conjugatedquadratures is ≳λ/4, e.g., ≳35 nm for λ=140 nm. For a spatial resolutionfor the measurement of the amplitudes of the conjugated quadratures of2.5 microns, a 1024×1024 pixel CCD, with a frame rate of approximately100/sec, and λ≳140 nm, a full scan of a 300 mm wafer can made at therate of approximately 10/hr for defects and/or artifacts withcharacteristic dimensions of the order of 35 nm on either a unpatternedor patterned wafer. The defects in or on a patterned wafer areidentified by comparing the measured array of conjugated quadratureswith a master array of conjugated quadratures generated by a masterpatterned wafer that does not have any defects. The information on themaster array of conjugated quadratures is stored on a memory device foraccess as required.

A full scan of a 300 mm wafer may be made at the rate of approximately10/hr for defects and/or artifacts with characteristic dimensions of theorder of ≳35 nm for λ≃250 nm although the resolution represented by thehigh spatial frequency of the measured conjugated quadratures is ≃65 nm.

The full scan of a 300 mm wafer made at the rate of approximately 10/hrfor defects and/or artifacts with characteristic dimensions of the orderof ≳35 nm may be of the surface of the 300 mm wafer or of an internalsection of the 300 mm wafer at a depth of the order of a micron.

The amplitudes and phases in an amplitude and phase representation ofthe conjugated quadratures measured in the first embodiment aresubsequently used determine properties and locations of defects andartifacts. The properties contain information about the size andcomposition of the defects and artifacts. The locations determined fromthe phases are interferometrically determined quantities and as suchpossess the advantages generally assigned to measuring displacementswith linear displacement interferometer.

A variant of the first embodiment comprises the same apparatus of thefirst embodiment except that pinhole array beam-splitter 12 is replacedwith an interference type beam-splitter and a pinhole array is placed atmulti-pixel detector 70. In other variants of embodiments of the presentinvention, pinhole array 12 is replaced by an array of microgratingssuch as described in cited U.S. Provisional Patent Application No.60/459,425 (ZI-50) and U.S. patent application Ser. No. 10/816,180,(ZI-50) filed Apr. 1, 2004 and entitled “Apparatus and Method for JointMeasurement Of Fields Of Scattered/Reflected Or Transmitted OrthogonallyPolarized Beams By An Object In Interferometry.”

In the first embodiment, multi-pixel detector 70 may comprise a frametransfer CCD that is configured such that one set of CCD pixel signalvalues may be generated and subsequently stored on the CCD wafer while aframe of a second set of CCD pixel signal values may be generated beforea readout of both the first and second set of the CCD signal values ismade. The time required to store the first set of CCD signal values isgenerally much less than the time required to readout a set of CCDsignal values for a frame transfer CCD. Thus, the advantage of the useof a frame transfer CCD is that the time between two consecutive pulsesof input beam 20 and the corresponding time between measurements ofelectrical interference signal values can be much less than when using anon-frame transfer CCD.

A second embodiment of the present invention is described that comprisesthe interferometric confocal microscopy system of the first embodimentoperated for joint measurement of conjugated quadratures using thebi-homodyne detection. In the second embodiment, beam-conditioner 22 isoperated to generate beam 24 comprising two frequency-shifted componentsor two frequency components which reference and measurement beamcomponents that relative phases that are shifted.

For generation of two frequency-shifted components of beam 24 whenoperating in the frequency shifted mode, the acoustic power toacousto-optic modulator 1120 (see FIG. 1 e) is adjusted so that theintensity of diffracted beam 1122 and the intensity of non-diffractedbeam 1124 are the same. The level of acoustic power in acousto-opticmodulator 1120 is controlled by signal 74 generated by electronicprocessor and controller 80.

The remaining description of the second embodiment is the same ascorresponding portions of the description given of the first embodimentof the present invention.

In other embodiments of the present invention, conjugated quadratures ofa high spatial frequency component are measured with Λ_(y)≅Λ_(x) (seeEquations (1) and (2)). In the other embodiments of the presentinvention, catadioptric imaging system 100 comprises two sections, i.e.,two pie sections, of catadioptric imaging system 210 oriented 90 degreesapart about the optic axis of catadioptric imaging system 100. Inaddition, beam 24 is further split by a beam-splitter (not shown in afigure) to generate reference and measurement beams for the planedefined by the orientation of the second section of catadioptric imagingsystem 210. The remaining description of the other embodiments is thesame as the description given for corresponding portions of thedescriptions of the first and second embodiments. The advantage of theother embodiments is the acquisition of information about defects and/orartifacts that have characteristic dimensions of the order of λ/2 in thex and y directions simultaneously.

In another embodiment of the present invention, the height h (see FIG. 2d) may be adjusted to be ≈λ/4. In that case, evanescent fields will beused to detect in the high speed scan sub-wavelength defects andartifacts in or on a substrate. The remaining description of theapparatus and method of the another embodiment is the same as thecorresponding portions of the description given for the first and secondembodiments of the present invention.

In yet other embodiments of the present invention, information isobtained about the properties of generation and propagation of signals,e.g., electrical, thermal, or acoustical, in a section of substrate 60by using a probe beam in addition to the measurement beam. Beam 24comprises a probe beam that precedes the measurement beam by a time τ.The probe beam and the measurement beam may have different opticalwavelengths. An advantage of the use of the yet other embodiments withrespect to the measurement of temporal response of the substrate is thehigh spatial frequency resolution in the plane of the section. Theremaining description of the yet other embodiments is the same as thedescription given for corresponding portions of the descriptions of thefirst and second embodiments of the present invention.

In other embodiments of the present invention, the first and secondembodiments are configured to make joint measurements of fields oforthogonally polarized beams scattered/reflected by the spots on or insubstrate 60 such as described in cited U.S. Provisional PatentApplication No. 60/459,425 (ZI-50) and U.S. patent application Ser. No.10/816,180, (ZI-50) filed Apr. 1, 2004 and entitled “Apparatus andMethod for Joint Measurement Of Fields Of Scattered/Reflected OrTransmitted Orthogonally Polarized Beams By An Object InInterferometry.” Also in the other embodiments of the present invention,a variant of the bi-homodyne detection method is used such as describedin cited U.S. Provisional Patent Application No. 60/459,425 (ZI-50) andU.S. patent application Ser. No. 10/816,180, (ZI-50) filed Apr. 1, 2004and entitled “Apparatus and Method for Joint Measurement Of Fields OfScattered/Reflected Or Transmitted Orthogonally Polarized Beams By AnObject In Interferometry” for ellipsometric measurements.

Other embodiments are within the following claims.

1. A method of detecting defects or artifacts on or in an object, saiddefects or artifacts characterized by a characteristic dimension, saidmethod comprising: generating an input beam for illuminating a spot at aselected location on or in the object, said spot having a size L that issubstantially larger than said characteristic dimension; deriving ameasurement beam and a reference beam from the input beam; directing themeasurement beam onto the object as an incident measurement beam thatilluminates said spot at that selected location on or in the object toproduce a backscattered measurement beam; interfering the backscatteredmeasurement beam with the reference beam to produce an interferencebeam, said reference beam being oriented relative to the backscatteredmeasurement beam so as to produce a peak sensitivity for a portion ofthe backscattered measurement beam that emanates from the object at apredetermined diffraction angle; converting the interference beam forthat selected location into an interference signal; and using theinterference signal for that selected location to determine whether anydefects or artifacts characterized by said characteristic dimension arepresent anywhere within a region on or in the object defined by the spotat that selected location.
 2. The method of claim 1, wherein L is atleast three times greater than the characteristic dimension.
 3. Themethod of claim 1, wherein L is at least an order of magnitude largerthan the characteristic dimension.
 4. The method of claim 1, wherein theincident measurement beam is at an angle of incidence θ_(I) with respectto a direction that is normal to the surface of the object, wherein thepredetermined diffraction angle is an angle θ_(D) relative to thedirection that is normal to the surface of the object, wherein thecharacteristic dimension is equal to Λ, wherein the incident measurementbeam is characterized by a wavelength λ, and wherein the diffractionangle θ_(D) is selected to satisfy the following relationship: Λ[sin(θ₁)−sin(θ_(D))]=λ.
 5. The method of claim 1, further comprisingperforming the steps of generating, deriving, directing, interfering,and converting for each of a sequence of different selected locations onor in the object, wherein the first-mentioned selected location is oneof said plurality of different selected locations.
 6. The method ofclaim 1, wherein generating the input beam involves generating a firstbeam at a first wavelength and a second beam at a second wavelength thatis different from the first wavelength, said first and second beamsbeing coextensive and sharing the same temporal window.
 7. The method ofclaim 6, wherein generating the input beam further involves for each ofa plurality of successive time intervals, introducing a correspondingdifferent shift in a selected parameter of the first beam andintroducing a different corresponding shift in the selected parameter ofthe second beam, wherein said selected parameters are selected from agroup consisting of phase and frequency.
 8. The method of claim 7,wherein using the interference signal to determine whether any defectsor artifacts are present comprises: for each of the plurality ofsuccessive time intervals, measuring a value of the interference signal;from the measured values of the interference signal for the plurality ofsuccessive time internals, computing the orthogonal components ofconjugated quadratures of fields of the backscattered measurement beam;and using the two computed orthogonal components of conjugatedquadratures of fields of the backscattered measurement beam to determinewhether any defects or artifacts characterized by said characteristicdimension are present within the spot.
 9. The method of claim 7, whereineach of said first and second beams includes a first component and asecond component that is orthogonal to the first component, wherein theselected parameter of the first beam is the phase of the secondcomponent of the first beam, and wherein the selected parameter of thesecond beam is the phase of the second component of the second beam. 10.The method of claim 7, wherein the selected parameter of the first beamis the frequency of the first beam, and wherein the selected parameterof the second beam is the frequency of the second beam.
 11. The methodof claim 1, wherein using the interference signal for that selectedlocation to determine whether any defects or artifacts are presentinvolves jointly measuring two orthogonal components of conjugatedquadratures of fields of the backscattered measurement beam.
 12. Themethod of claim 1 implemented in an inspection tool wherein the objectis a mask.
 13. The method of claim 1 implemented in an inspection toolwherein the object is a reticle.
 14. The method of claim 1 implementedin a lithography tool wherein the object is a wafer.
 15. The method ofclaim 1 implemented in a lithography tool wherein the object is a waferstage and the artifacts are alignment marks.
 16. A method of detectingdefects or artifacts on or in an object, said defects or artifactscharacterized by a characteristic dimension, said method comprising:focusing an incident measurement beam to a spot that illuminates atarget area on or in the object to produce a backscattered measurementbeam, said spot having a size L that is substantially larger than saidcharacteristic dimension; interfering the backscattered measurement beamwith a reference beam to produce an interference beam, said referencebeam being oriented relative to the backscattered beam so as to producea peak sensitivity for a portion of the backscattered beam that emanatesfrom the object at a predetermined diffraction angle; converting theinterference beam to an interference signal; and determining from theinterference signal whether any defects or artifacts characterized bysaid characteristic dimension are present within the target area.
 17. Amethod of detecting defects or artifacts on an object, said defects orartifacts being characterized by a characteristic dimension, said methodcomprising: generating a reference beam; generating a measurement beamfor illuminating a spot on or in the object, said spot having a size Lthat is substantially larger than said characteristic dimension; andusing the measurement and reference beams to interferometricallydetermine whether any defects or artifacts characterized by saidcharacteristic dimension are present on or in the object.